The invention relates to optical networking.
An optical add drop multiplexer (OADM) is a device used to extract a set of optical signals (also called xe2x80x9cwavelengthsxe2x80x9d herein) from a wavelength division multiplexed (WDM) signal input to the OADM, and to subsequently reinsert the extracted set of wavelengths output by the OADM. FIG. 1 (prior art) shows a block diagram of an OADM 100. A WDM optical signal, (also called xe2x80x9cwavelength bandxe2x80x9d herein) comprises a plurality of wavelengths. One of the wavelengths, xcex2, is extracted (also called xe2x80x9cdroppedxe2x80x9d herein) from an input line side 102 via a de-multiplexer 104, and then subsequently reinserted (also called xe2x80x9caddedxe2x80x9d herein) onto an output line side 108 via a multiplexer 106. The purpose of adding and dropping wavelength(s) in this manner is to obtain information encoded on the dropped wavelength (in FIG. 1, the dropped wavelength is xcex2); one may also transmit new information on dropped wavelengths. In most instances the carrier wavelength of the dropped wavelength is the same as carrier wavelength of the added wavelength.
It is well recognized that a preferred implementation of an OADM employs tunable de-multiplexers and multiplexers, enabling choice in the wavelength(s) to be dropped/added. However, until tunable filter technology is mature, OADMs will continue to be constructed using fixed de-multiplexers and fixed multiplexers, along with switches to select the wavelength(s) to be dropped.
There has been an evolution of OADM products in recent years. The first generation OADMs, as in FIG. 1, were not configurable, i.e., the determination of which wavelengths are dropped and added was set at the time of manufacture of the OADM. Second generation OADMs are configurable, but have a number of limitations. FIG. 2 (prior art) shows a simple three-wavelength system utilizing a second generation OADM 200. The second generation OADM 200 de-multiplexes all the wavelengths of interest via a de-multiplexer 204, and passes each wavelength through a respective 2xc3x972 switch 206 that is controllable by the user. The 2xc3x972 switch 206 has two settings: in one setting the 2xc3x972 switch 206 allows the demultiplexed wavelength to pass through with no further processing, via an optical waveguide 212, while in the other setting the 2xc3x972 switch 206 connects the demultiplexed wavelength, e.g., xcex1, to a drop optical waveguide 208, and simultaneously connects an add optical waveguide 210 to the optical waveguide 212 that is attached to a multiplexer 214, which multiplexer multiplexes all wavelengths input to the multiplexer, for example, wavelengths xcex1, xcex2, xcex3, producing a multiplexed signal to an output via an egress line 216.
The system depicted in FIG. 2 has a number of disadvantages:
(1) Poor optical performance: Due to the filtering characteristics of each successive OADM, a series of OADMs that are concatenated typically results in channel narrowing.
(2) Poor network performance during upgrade or provisioning activities: Expanding the number of wavelengths in the system and/or expanding or changing the number of wavelengths added/dropped at a node typically requires shutting the system down for modification.
(3) High cost. For a system with a large number of wavelengths, it is costly to de-mulitplex, switch, and multiplex all wavelengths.
(4) Lack of flexibility. To avoid the high cost incurred as explained above, this type of implementation forces the user to pick a subset of wavelengths, thereby having reduced flexibility in add/drop capabilities.
(5) Inventory. Reducing the wavelength set addressed by any one OADM, as discussed in (4), requires the operator to stock a number of different types of OADM units (also called xe2x80x9cmodulesxe2x80x9d herein), leading to excessive inventory management.
What is needed is an OADM that overcomes the drawbacks enumerated above.
Methods and apparatus are contemplated for dropping and adding a user-chosen wavelength sub-band in a band of wavelengths. Methods and apparatus are contemplated for separating and dropping individual wavelengths from a wavelength sub-band, and adding the individual wavelengths back to the wavelength sub-band. An OADM is contemplated that combines a wide-band OADM stage and a narrow-band OADM stage.